Researchers at Columbia University found only one period, the Paleocene-Eocene Thermal Maximum (PETM), a period approximately 55 to 56 million years ago, that had such rapid acidic change. Signs of the PETM are still visible as brown mud layer in ocean sediment cores that are flanked by thick deposits of white plankton fossils, the researchers said. During the period, a surge in carbon concentration in the atmosphere increased the temperature of Earth and "turned the oceans corrosive".
According to the research published, carbon dioxide in the air reacts with seawater and creates carbonic acid, which is neutralized over time by fossil carbonate shells on the seafloor. However, too much carbon can deplete the carbonate ions that are needed by organisms such as corals, mollusks and some plankton for shell construction. Oversupply of carbonic acid can therefore result in the loss of corrals, oysters and salmon, Baerbel Hoenisch, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory said.
During a period of 5,000 years of the PETM - some claim it may have lasted for as long as 20,000 years - atmospheric carbon doubled to 1,800 parts per million (ppm), and average global temperatures rose by about 6 degrees Celsius, causing about on half of all one-cell organisms on the sea floor to disappear, which may have taken more complex life forms with them as well. Over the past 100 years, the scientists estimate that the pH value of the oceans has fallen by about 0.1, which is a rate that is believed to be 10 times faster than it happened during the PETM.
The findings have led the researchers to create lab tests to eventually predict the effects of the current acidification of oceans. However, they noted that there are too many variables (high carbon dioxide, water temperature, reduced ocean pH, dissolved oxygen levels ) at play and a forecast is "difficult.